The reduction in friction performance in lubricants has been pursued in the industry for a number of years. Because of environmental concerns, industry more recently is focusing on enhancing the fuel economy benefits over extended periods of oil use.
U.S. Pat. No. 4,178,258 teaches a lubricating oil for use in spark ignition and compression ignition engines which exhibits enhanced antiwear and friction characteristics by containing an antiwear amount of a molybdenum bis(dialkyl dithiocarbamate). The lubricant is described as being especially effective in reducing wear and friction if the lubricant also contains a zinc dialkyldithiophosphate (ZDDP).
U.S. Pat. No. 4,395,434 teaches an antioxidant additive combination for lube oils prepared by combining a sulfur containing molybdenum compound prepared by reacting an acidic molybdenum compound, a basic nitrogen compound and carbon disulfide with an organic sulfur compound. The organic sulfur compound is described as including metal dialkyldithiophosphates, and metal dithiocarbamates, among other organic sulfur compounds.
U.S. Pat. No. 4,529,526 teaches a lubricating oil composition comprising a base oil and a sulfurized oxymetal organic phosphorodithioate and/or a sulfurized oxymetal-dithiocarbamate and at least one zinc alkylcarbyl dithiophosphate, along with a calcium alkybenzene or calcium petroleum sulfonate and an alkenylsuccinic acid imide.
U.S. Pat. No. 4,786,423 teaches an improved lubricant which contains a mineral or synthetic base stock oil and two heavy metal compounds as well as a metal and sulfur free phosphorous compound. The heavy metal compounds can be molybdenum dithiocarbamate in combination with zinc dialkyldithiophosphate. The other phosphorous compound can be trialkyl or triaryl phosphate. The lubricant is prepared by, for example, heating the base stock to between room temperature and about 100.degree. C. for two hours, then adding the subsequent components to the heated oil approximately 20 minutes apart under the referenced elevated temperature.
WO 95/19411 (PCT/US95/00424) is directed to additives for lubricants which are combinations and reaction products of metallic dithiocarbamates and metallic dithiophosphates. The preblended combinations and reaction products are described as showing good stability and compatibility when used in the presence of other commonly used additives in grease or lubricant compositions. The metals of the metal dithiophosphates and metal dithiocarbamates may be selected from nickel, antimony, molybdenum, copper, cobalt, iron, cadmium, zinc, manganese, sodium, magnesium, calcium and lead. The combination and reaction products are described as providing enhanced friction reducing and anti-wear properties at extreme pressure. Additional anti-oxidation, cleanliness, anti-fatigue, high temperature stabilizing and anti-corrosion properties are also described as potentially present. The metallic dithiocarbamate and metallic dithiophosphate are mixed, generally at any suitable conditions with temperatures varying from -20.degree. C. to 250.degree. C., preferably between 50.degree. C. and 150.degree. C. Reaction rather than blending will usually occur if the temperature is between 70.degree. C. and 100.degree. C. The metallic dithiocarbamates and the metallic dithiophosphates may be combined in any ratio from 1:9 to 9:1. In the Examples, reaction temperatures of only 80.degree. C. to 100.degree. C. were employed.
U.S. Pat. No. 4,812,246 teaches a lubricating composition comprising a particular base oil and additives comprising a phenol based antioxidant and/or organomolybdenum compounds such as molybdenum dithiocarbamate. The lubricating composition can also contain other common additives such as zinc dialkyl dithiophosphates, etc.
M. Meienberger, et al., Inorganica Chimica Acta 213, p. 157-169 (1993) discloses the reactions of certain (Mo.sub.3 S.sub.7 L.sub.3).sup.+4 compounds.
Copending U.S. application Ser. No. 766,828, filed Dec. 13, 1996 discloses a method for making a lube oil composition using a different reaction product, i.e., the reaction product of molybdenum dialkyl dithiocarbamate and metal dihydrocarbyl dithiophosphate. Disadvantageously this reaction forms a metal precipitate which must be separated from the product before use.
Due to environmental concerns and Corporate Average Fuel Economy ("CAFE") requirements, the industry is placing increasing emphasis not only on the initial fuel economy performance of engine oils, but also on the retention of the performance during oil use. Certain molybdenum friction modifiers are known to offer frictional benefits, which, however, degrade as the oil ages (K. Arai et. al., "Lubricant Technology To Enhance The Durability Of Low Friction Performance Of Gasoline Engine Oils", SAE 952533 (1995)). It would be desirable to have an engine oil with improved friction performance and friction retention properties. Applicants' invention addresses these needs.